When a blank is formed into a final shape by press forming, an improvement of yields or an improvement of formability is an important issue.
Generally, to improve yields, it is preferable to adjust the forming conditions to as close as the conditions for stretch forming by minimizing the flow rate at which a material flows into a mold set during forming. However, an excessively low flow rate of the material would cause a shortage of the material in the mold set during forming, whereby the blank would be excessively thin and would have defects such as cracks. When, on the other hand, forming mainly including drawing were performed in order to prevent cracks, yields would be lowered. In order to address such problems, various measures have been taken thus far.
Patent Literature 1 has disclosed a method for improving yields by reducing excess metal as a result of reserving, in the early stage of press forming, a range in which a blank is not restrained. Patent Literature 2, on the other hand, has disclosed a method for preventing forming defects by allowing a local area of a press die set to drive as an individual movable punch and performing forming using the movable punch after placing a blank in the mold set in advance.